How to change volume on sub woofer missing its control speaker?

So, I have a sub woofer that IS NOT broken or blown, but when I plug in my music player into it, I BARELY hear it when the player is on max volume. I think this is because it was a 3 piece set (2.1) with the volume and control knob on one of the speakers that went with it (I don't have to other two speakers, just sub). So, how can I get the volume adjusted to a hearable level? It was pretty cheap and I can open it up if necessary.